This invention relates to power plant operations, and, more particularly, to an approach for removing particulate matter from a flue gas stream produced in a fossil fuel power plant, especially a coal-fired power plant.
In a fossil fuel power plant, a fuel is burned in air to produce a flue gas. The flue gas heats water in a boiler to generate steam, which turns a turbine to produce power. After passing through various apparatus, the flue gas is exhausted through a stack to the atmosphere.
The flue gas includes solid particulate matter and a variety of gaseous contaminants. The maximum permissible emission levels of the particulate matter and gaseous contaminants are set by laws and regulations. The maximum emission levels are typically far less than the amounts present in the flue gas as it is produced, and various types of gas treatment apparatus are usually provided to reduce the particulate matter and gaseous contaminants in the flue gas before it leaves the stack.
In most fossil fuel power plants, particulate matter in the gas stream is removed by electrostatic precipitation. An electrostatic charge is applied to the particulate matter in the flue gas, and the flue gas passes between charged electrodes. The particulate matter is deposited upon the electrode having the opposite charge to that of the particulate and is later removed.
The fuel typically contains from about 0.5 percent to about 6 percent sulfur, which at least in part oxidizes to sulfur dioxide during combustion. A small part of the sulfur dioxide further oxidizes to sulfur trioxide. Since the combustion air and the fuel also contain moisture, the flue gas contains water vapor. The sulfur trioxide and water vapor in the flue gas react to produce sulfuric acid, which deposits upon the particulate matter. The sulfuric acid deposited upon the particulate matter imparts a degree of electrical conductivity to the particulate and promotes the electrostatic precipitation process.
If the fossil fuel contains too little sulfur so that there is a deficiency of sulfur trioxide and thence sulfuric acid in the flue gas, the electrostatic precipitator cannot function properly because of the high electrical resistivity of the particulate. It is therefore known to add sulfur trioxide from another source to the flue gas produced from burning low-sulfur fossil fuels. See, for example, U.S. Pat. No. 2,993,429.
In such a sulfur trioxide conditioning system, sulfur is burned to form sulfur dioxide, which is passed over a catalyst to achieve further oxidation to sulfur trioxide. The sulfur trioxide is injected into the flue gas flow upstream of the electrostatic precipitator. The amount of injected sulfur trioxide is controlled by varying the amount of sulfur that is burned.
Sulfur trioxide injection systems work well, and are widely used. In some instances, however, there are drawbacks. The catalytic conversion of sulfur dioxide to sulfur trioxide is not completely efficient, and additional sulfur dioxide is added to the flue gas flow. A constant supply of sulfur feedstock is required, and this feedstock must be safely handled. The several components of the burning, catalyzing, and injecting system must be kept in good working order, and there is a power consumption associated with the process.
U.S. Pat. No. 5,011,516 describes an alternate approach in which a slip stream of flue gas is drawn from the main flow and passed over a catalyst. A portion of the sulfur dioxide in the slip stream is oxidized to sulfur trioxide, and the slip stream is merged back into the main flue gas flow. While of interest, this approach has major drawbacks when implemented. System thermal efficiency is reduced because less heat is recovered. There is typically insufficient mixing of the slip stream and the main flow at the point where they rejoin due to an insufficient pressure differential.
Moreover, the '516 patent does not disclose any approach which permits control of the amount of sulfur trioxide produced, responsive to variations in the sulfur content of the fuel and changes in other operating parameters. A patent to a related approach, U.S. Pat. No. 3,581,463, suggests using a fan to draw a portion of the hot gas flow into the slip stream, but gives no further details as to how the amount of sulfur trioxide can be controlled. One can imagine that valving could be added to the slip stream to control its total flow, but such valves are complex, expensive, and difficult to build.
There is therefore a need for an improved approach to sulfur trioxide conditioning of flue gas streams. The present invention fulfills this need, and further provides related advantages.